Asphalt is typically used to pave roads, bridges, racetracks, airport runways and taxiways, parking lots, bicycle paths, loading docks, and the like to provide surfaces for accommodating vehicular traffic. In the 1980's, asphalt was used to provide surfaces for vehicular traffic as layers of varying composition. Subsequent systems employed single layers of asphalt. In the single-layer systems, virgin asphalt was typically mixed with aggregate at a temperature of 130 to 190 degrees C.
The asphalt used in paving is a solid or semisolid bituminous material that is either naturally occurring or derived from petroleum refining processes and includes paraffinic and aromatic hydrocarbons and heterocyclic compounds. Aggregate such as crushed stone is typically incorporated into the asphalt. The most common type of asphalt used as a paving composition in the United States is hot mix asphalt (HMA) of which there is several variations or job mix formulations.
In one variation of HMA, an additive such as a polymer is added to an asphalt binder, the combination in turn being added to the asphalt, thereby resulting in a polymer modified binder (PMB). The PMB is then combined with aggregate in a facility to produce the HMA. The polymer facilitates the formation of a cohesive structure that affords a degree of flexibility to the asphalt to accommodate creep and stress. In this polymer modified HMA, the resulting mixture has the ability to accommodate creep or stress relaxation at low temperatures to minimize the potential for thermal and reflective cracking.
The PMB is generally shipped in tankers and stored in pre-heated containers at constant temperature before being combined with aggregate. PMB sometimes will require repeated cycles of agitation to maintain the PMB in a state of readiness for subsequent incorporation with aggregate and application as a paving material. The need for storage in this manner results in the need for HMA facilities to have additional tank storage space. In addition, complexities result if only small quantities of polymer modified HMA are required. In these instances the HMA facility may incur additional charges for a partial load of PMB or may opt to “waste” some premium PMB in a lower quality product since a minimum quantity of material is required in the tank for production of the polymer modified HMA to occur.